1. Field of the Invention
The present invention relates in general to in-circuit test apparatus, and more particularly to an in-circuit test apparatus for detecting reverse insertion of capacitors into a PCB board to prevent a faulty operation of a circuit.
2. Description of the Prior Art
Referring to FIG. 1, there is shown a block diagram of a conventional in-circuit test apparatus. As shown in this drawing, the conventional in-circuit test apparatus comprises a constant current source 1 for supplying a constant current I1, a relay circuit 2 for transferring the constant current I1 from the constant current source 1, and a test board 3. Capacitors C1 and C2 are inserted into the test board 3 and charge with the constant current I1 transferred by the relay circuit 2.
A voltage follower 4 is provided in the conventional in-circuit test apparatus to amplify charging voltages V1 and V2 on the capacitors C1 and C2 to prevent the charging voltages V1 and V2 from being discharged due to internal resistances of the capacitors C1 and C2 according to a state of the relay circuit 2.
The conventional in-circuit test apparatus also comprises an amplifier 5 for amplifying an output voltage from the voltage follower 4, a sample/holder 6 for sampling an output voltage from the amplifier 5 at a predetermined period and holding the sampled voltage, an analog/digital (A/D) converter 7 for converting an output voltage from the sample/holder 6 into a digital signal, and a microprocessor 8 for sensing capacitances of the capacitors C1 and C2 in response to the digital signal from the A/D converter 7.
The relay circuit 2 includes a relay RY1 for transferring the constant current I1 from the constant current source 1 to the capacitors C1 and 02 and relays RY2 and RY2' for switching the constant current I1 which is transferred from the relay RY1 to the capacitots C1 and C2, to charge the constant current I on the capacitors C1 and C2.
The operation of the conventional in-circuit test apparatus with the above-mentioned construction will hereinafter be described.
First, the operation of sensing the capacitance of the capacitor C1 normally inserted into the test board 3 will be mentioned.
The constant current I1 is supplied from the constant current source 1 and the relays RY1 and RY2 in the relay circuit 2 are turned on. In this case, the constant current I1 from the constant current source 1 is transferred to the capacitor C1 in the test board 3 by the relay RY1. The transferred constant current I1 is charged on the capacitor C1 for a predetermined time period. The charging voltage V1 on the capacitor C1 can be expressed by the following equation: EQU V1=(I1.times.T)/C1
At that time that the charging of the capacitor C1 is completed, the relay RY1 in the relay circuit 2 is turned off and the charging voltage V1 on the capacitor C1 is then amplified by the voltage follower 4. The output voltage from the voltage follower 4 is amplified by a predetermined level by the amplifier 5 and then applied to the sample/holder 6. The sample/holder 6 samples the output voltage from the amplifier 5 at the predetermined period and holds the sampled voltage.
The output voltage from the sample/holder 6 is converted into the digital signal by the A/D converter 7 and then applied to the microprocessor 8. In response to the digital signal from the A/D converter 7, the microprocessor 8 senses the capacitance of the capacitor C1.
Then the operation of sensing the capacitance of the capacitor C2 reversely inserted into the test board 3 will be mentioned.
In a similar manner to that in the operation of sensing the capacitance of the capacitor C1, the constant current I1 is supplied from the constant current source 1 and the relays RY1 and RY2' in the relay circuit 2 are turned on. As a result, the constant current II from the constant current source 1 is transferred to the capacitor C2 in the test board 3 by the relay RY1. The transferred constant current I1 is charged on the capacitor C2 for the predetermined time period. The charging voltage V2 on the capacitor C2 can be expressed by the following equation:
V2=(I1.times.T)C2
When the charging of the capacitor C2 is completed, the relay RY1 in the relay circuit 2 is turned off and the charging voltage V2 on the capacitor C2 is then amplified by the voltage follower 4. The output voltage from the voltage follower 4 is amplified by the predetermined level by the amplifier 5 and then applied to the sample/holder 6. The sample/holder 6 samples the output voltage from the amplifier 5 at the predetermined period and holds the sampled voltage.
The output voltage from the sample/holder 6 is converted into the digital signal by the A/D converter 7 and then applied to the microprocessor 8. In response to the digital signal from the A/D converter 7, the microprocessor 8 senses the capacitance of the capacitor C2.
However, the conventional in-circuit test apparatus has a disadvantage in that only the capacitances of the capacitors (for example, electrolytic) are sensed regardless of the normal or reverse insertion into the test board. That is, the conventional in-circuit test apparatus fails to detect whether capacitors have been inserted in a test board with a correct polarization or an incorrect polarization. The condition of a capacitor being inserted on a test board with an incorrect polarization will hereafter be referred to as reverse insertion. In the case where a circuit is operated for a long time under the condition that the capacitor remains at its reverse insertion state, components of the circuit may be subjected to a damage, resulting in a faulty operation of the circuit. The faulty operation of the circuit exerts a bad influence on the associated equipments.